Dragline buckets are shovel-like enclosures which are advanced and controlled by flaccid lines, such as chains, cables or ropes. The buckets include an open end through which earthen material is received and accumulated as the buckets are dragged through the ground.
In a typical prior art system, a dragline bucket 2 is supported generally at a rear portion thereof by a pair of hoist lines 4 attached to the opposed side walls of the bucket. The upper ends of the hoist lines are secured to a dump block connecting link member 16 by swivel links 18. A spreader bar 5 is placed between the two hoist lines to prevent the lines from rubbing on the sides of the bucket. The upper end of link member 16 is attached to a hoist link 20 which, in turn, is connected to an equalizer 22. The equalizer is secured to the lower ends of lift lines 26 by hoist sockets 24. The lift lines are coupled to an overhead boom (not shown) for lifting and lowering the bucket. Equalizer 22 rotates relative to hoist link 20 to account for any differences which may exist in the lengths of the two lift lines.
A dump line 6 is provided to support and control the front end of bucket 2. One end of the dump line is attached by a socket 8 to a front upper edge of the bucket 2. The other end of dump line 6 is secured by a socket 13 to a drag line 7, which is typically referred to as a drag rope. The drag line 7 is connected to the front of the bucket by a pair of pull lines 9, typically referred to as drag chains, in order to pull the bucket through the ground in a digging operation.
The dump line is supported along a medial portion by a dump block 10, which is secured by a dump link 14 to an outwardly extending arm 16a of link member 16. Dump block 10 includes a rotatable sheave 12 about which dump line 6 is passed, and a housing 11 which is used to attach the sheave to link member 16. The tension applied to the dump line by its connection to drag line 7 is used to lift the front of the bucket and thereby prevent unwanted forward tipping of the bucket. Release of the tension in the drag line 7 permits the bucket to tip forward and the accumulated load to be dumped.
As can be appreciated, the forces encountered in a dragline bucket operation are typically very large. As an example, the forces experienced in the lift lines for a large dragline bucket can reach 500,000 pounds or more. As can be appreciated, hoist lines 4, lift lines 26, and dump line 6 via dump block 10 all transmit large forces with vertical and horizontal components to the hoist linkage assembly, and particularly to link member 16. These large and disparate forces translate into large moment forces which impose additional substantial stresses on the parts of the hoist linkage assembly. These additional stresses work to limit the usable life of the various parts. Such reductions in the usable life of these parts not only results in additional replacement costs, but also increased down time in the operation of the bucket.
Moreover, conventional hoist linkage assemblies for dragline buckets have typically included many components which collectively add a significant weight to be supported by the boom. This weight of the hoisting assembly causes a concomitant reduction in the amount of load which can be carried by the bucket because of weight limitations of the boom and other related equipment. This collection of parts in the hoisting linkage also has a significant length which limits the distance the bucket can be pulled up to the boom, and hence, the height to which the bucket can be raised.